Re: [LEAPSECS] What problems do leap seconds *really* create?
On Wed, Jan 29, 2003 at 03:43:02PM -0700, Rob Seaman wrote:
> Ken Pizzini says:
> > I realize that the astronomical community has evolved to a consensus
> > that UT1 (approximated by UTC) is a highly useful way to mark time,
>
> Rather we've evolved a consensus that different problems require
> different systems of time - not surprising, since we invented most
> of them.
Right, but in its way UT1 is "king" because that is the measure of
earth-position time which is used in the definition of our current
time standard, UTC.
> > with the additional feature that it is usable as a civil time standard,
>
> It isn't just usable - it is preferable to many alternatives.
That is what I was saying: of the useful earth-position based ways
to mark time, UT1 was felt to be among the best, if not the best,
choice when considerations of civil time were thrown into the mix.
Especially if one is convinced that the prime meridian shall remain
special (even if the land masses currently under it might happen to
suffer from tectonic drift over time).
> > but there is so much of that evolution which is based on historical
> > accident rather than purely technical requirements
>
> "Historical accident" makes it sound like the practice of timekeeping
> was some afterthought to events.
You mean to tell me that our systems of timekeeping were inspired
solely by dispassionate rational thought about what good designs for
timekeeping would be, without any historical baggage? That our current
practice of breaking a day into 24 hours is inspired by something
other than the history of how sub-day measurement was handled by
pre-mechanical-clock cultures, or that the further splitting of hours
into 60 "minute" (read that as "my-newt") parts and that still further
into 60 secondary parts has nothing to do with the vagaries of history?
Maybe it is just my use of the term "historical accident" is not
a broadly used idiom: by it I mean that, especially in the context
of some variety of evolution, it is easy to think up turns in the
course of history, unrelated to any intrinsic aspect of an entity
under consideration, which would give that entity a significantly
different character than it currently has. For example, the fact
that UTC's meridian passes through London, rather than through, say,
Paris or Washington, has to do with the politics of how GMT came
to be a global time-and-longitude standard, and not because of any
technically intrinsic merit of that meridian over the others.
> The reality is that timekeeping has
> often been central to other, bloodier, battles - from Augustus Caesar
> appropriating an extra day from February into "his" month
If that is not a "historical accident" I don't know what is...
Our calendar, with its system of 31,28,31,30,31,31,30,31,30,31 day
months, and leap year day being added to the second month of that list,
has a full and rich history. Some of that history relates to attempts
to improve the calendar dramatically (Julius Caesars reforms come to
mind), some relates to purely political tinkering (such as Augustus'
grab), and some relates to trying to make incremental improvements
without major reworking (such as the reform of leap year rules
introduced under pope Gregory).
The reformationists of the French Revolution attempted to make a
"more rational" calendar as part of their overall scheme which has
given us the metric system, but (for various reasons) that aspect of
their new way of measuring failed. One lesson that has been suggested
from that is that a good design for a timekeeping standard _must_
consider historical and cultural factors. That is no doubt why
everyone I've seen speak out in this forum tries to stick with units
of SI seconds, with different varieties of "days", and bandies about
86400 as a magic number, rather than going off on quixotic suggestions
of things like decimal clocks or reform of how our calendar marks the
relation between days and years. (I'm sure that there are several
of us who would _like_ to see such dramatic reforms, but they are
both well outside any reasonable interpretation of what this list's
charter is and so far out of the mainstream of our culture that such
reforms are bound to fall flat.)
> to Harrison's
> chronometer that was instrumental to the building of a later empire.
Ah, and this ties in to one of the most contentious points that I've
seen discussed on this list: historically, in the days before crystal
oscillators and atomic clocks, good determination of longitude has
been inextricably tied to the good determination of time. Some appear
to feel that this history is important to preserve in our civil time
standard ("UT1 rules!" "UTC ain't broke"); others appear to feel
that it is irrelevant ("Just use TAI, dammit!").
> Is there nobody on this list who was present at the birth of UTC?
Alas, I was not. (Well, in the sense of "was born" I "was present",
but not in the sense of "participated in or observed the process,
or was aware of the result and its implications shortly after its
birth". I know that by 1974 I was aware that the bodies responsible
for maintaining the timekeeping standards were adding leap seconds
now and again, but it wasn't until the mid-1980s that I had any
appreciation of what that meant in practical terms.)
> It is a good, solid, pragmatic standard.
Yes, for what it does. It gives 0.9s-error-bound approximation of
UT1 (useful as a starting point for computing all kinds of other
earth-positional-based timescales), it is a good fit for a civil time
standard (with a stable meridian, which it appears that some feel is
either an additional bonus or an essential nod to history), and it
is suitable for implementation with clocks which tick SI seconds.
I wholeheartedly grant that UTCs recognition and reconciliation of
these goals is a significant achievement, but I still ask "are those
the right goals, and if so, why?"
> The ability to issue leap
> seconds monthly clearly represents a recognition that these would be
> needed to preserve the standard over hundreds or thousands of years.
The issuance of leap seconds is a technical mechanism which enables
the UTC clock to tick SI seconds while maintaining a 0.9s error bound
on its difference of UT1. Those two requirements dictate that even
for any short term use that there must be some way of introducing
adjustments, and leap seconds are one of the most natural solutions
to reconciling these requirements. The only aspect of UTC which
can be construed as enabling "hundreds of thousands of years" of
preservation of the standard is the fact that the standard allows
for leap seconds to occur as often as once per calendar month despite
the lack of current need for such frequent adjustment, but (second-
guessing here) I suspect that the reason for the allowance of monthly
adjustments had more to do with uncertainty about the reliability of
the early 1970s model of the earth's rotation than it did with a
grand plan to make the standard usable for such a long span of time.
My biggest quibble with UTC is that the value of the specific set of
criteria that it was designed to meet never seem to be spelled out.
Some of its design choices seem reasonable in light of the state
of the art when it was created (the 0.9s error bound on UT1 was no
doubt ample for users at that time; early IERS data shows that, at
the time, this bound was only slightly larger than was required to
ensure that the promise made by the standard could be fulfilled),
but seems rather lax by contemporary standards; other design
choices appear to be made because of tradition (associate civil
time with a fixed meridian; this is also tied in to why |UTC-UT1|
is the constraint of interest, rather than some other |UTC-XXX|),
though that is just a surmisal, not something I see spelled out; and
finally, the constant-length seconds appears to be a concession to
the fact that it is very challenging to make real-time determination
of UT1, and it is much more realistic to use high-precision clocks
to fill in the gaps between IERS pronouncements than to insist on a
time standard which directly broadcasts UT1, but again I never see
this quite spelled out (though I have seen several quick hand-waves
pointing in this direction). Maybe I am just failing to look in the
right places; I certainly have not come across a description of the
decision making that went into the creation of UTC in my wanderings.
> > that I find it hard to believe that there would be no possible way to
> > improve upon it, even after non-astronomical constraints are factored
> > in, if only it were possible to start anew with a clean slate.
[...]
> What we object to (if my friends don't mind my saying so) is not the
> idea of *improving* UTC. What we object to is this current process
> which appears to be an attempt to discard the standard entirely -
> and to do so with minimal consent.
Excellent! So I guess maybe my question _should_ be: is there
a better forum for the discussion of how to achieve improved time
standards than LEAPSECS? From the last sentence I quoted above I gather
that some may feel that the original impetus for this list --- a
rather abrupt desire for a fast-track "fix" for how leap seconds are
handled --- is the extent of what is the proper subject matter for
this list, and talk about long-term redesigns of time standard(s)
(whether unlikely dramatic changes or more probable minor tweaks)
and dissemination mechanisms for such standards is really just a
tangential topic that ought to move elsewhere.
> Please! Let's talk about ways to improve UTC and civil timekeeping.
> And let's take the appropriate amount of time to reach a decision -
> say - 40 or 50 years. In the mean time, let's pay attention to the
> real question, which is how to build an infrastructure that will
> dramatically improve the dissemination of all time signals.
40 or 50 years sounds like a plausible time frame for the full phasing
in of a new system, but I'd really hope that it would take less than 10
years to reach a decision of what that system would be...
[snip of a section which I don't feel a need to add anything to.]
> Leap seconds are discussed very prominently in a very short document.
Are you referring to the GPS World article which triggered the current
discussion, or some other document that I should know about?
--Ken Pizzini
Received on Thu Jan 30 2003 - 00:29:29 PST
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